The present invention relates to printer devices, and particularly, although not exclusively, to a method and apparatus for determining and correcting misalignments between printheads in ink jet devices.
It is known to produce paper copies, also known as xe2x80x9chardxe2x80x9d copies of files stored on a host device, e.g. a computer using a printer device. The print media onto which files may be printed includes paper and clear acetates for use in lectures, seminars and the like.
Referring to FIG. 1, there is illustrated a conventional host device 1, in this case a personal computer, linked to a printer device 2 via a cable 3. Amongst the known methods for printing text or graphics and the like onto a print media such as paper it is known to build up an image on the paper by spraying drops of ink from a plurality of nozzles.
Referring to FIG. 2, there is illustrated schematically part of a prior art printer device comprising an array of printer nozzles 4 arranged into parallel rows. The unit comprising the arrangement of printer nozzles is known herein as a printhead. In a conventional printer of the type described herein, the printhead 5 is constrained to move in a direction 6 with respect to the print media 7 e.g. a sheet of A4 paper. In addition, the print media 7 is also constrained to move in a further direction 8. Preferably, direction 6 is orthogonal to direction 8.
During a normal print operation, printhead 5 is moved into a first position with respect to the print media 7 and a plurality of ink drops 9a, 9b are sprayed from a number of printer nozzles 4 contained within printhead 5. This process is also known as a print operation. After the completion of a print operation the printhead 5 is moved in a direction 6 to a second position and another print operation is performed. In a like manner, the printhead 5 is repeatedly moved in a direction 6 across the print media 7 and a print operation performed after each such movement of the printhead 5. In practice, modern printers of this type are arranged to carry out such print operations while the printhead is in motion, thus obviating the need to move the printhead discrete distances between print operations. When the printhead 5 reaches an edge of the print media 7, the print media is moved a short distance in a direction 8, parallel to a main length of the print media 7, and further print operations are performed. By repetition of this process, a complete printed page may be produced in an incremental manner.
Since the advent of colour printing, printers with more than one printhead are typically used. Generally, four printheads are used, each storing and printing a different colour; for example: cyan; magenta; yellow; and black. The inks from the four printheads are mixed on the print media to obtain any other particular colour.
However, full colour printing requires that the inks from the individual printheads are accurately applied to the print media.
In order that this may be achieved, precise alignment of the various printheads is required. The mechanical misalignment of a printhead may result in an offset in the positioning of ink drops on the print media. Such offsets may occur in the X direction (in the media advance/media axis) or the Y direction (in the carriage/scan axis). Additionally, angular offsets may also arise. If each printhead in a printer is not sufficiently accurately aligned with the remaining printheads of the printer, a misregistration between the images formed by the different coloured ink drops on the print media may result. This may cause too much ink to be deposited in some areas and too little ink to be deposited in others. This often gives rise xe2x80x9cgrainyxe2x80x9d appearance in the printed image. This type of print error is often particularly noticeable to the viewer. Consequently, such misregistrations are generally unacceptable, with colour printing typically requiring image registration accuracy from each of the printheads of {fraction (1/2400)} inch.
Various systems have been devised to address misregistration. In particular, systems have been devised in order to ensure that offsets in the X direction (media axis) are reduced to acceptable levels. One such known system employs a unitary colour printhead, which contains the nozzles of each ink colour: cyan; magenta; and yellow. Thus, the nozzles of each ink colour may be accurately aligned with those of the other colours on manufacture. Thus, when the printhead is mounted in the print carriage of a printer, the positions of the nozzles of each ink colour are constrained with respect to each other. In this way, the operator need only ensure that the colour printhead is correctly aligned with the black ink printhead.
In this system, this is achieved by printing two overlying alignment patches on the print medium, one with the black ink printhead and the other with the colour printhead. Each alignment patch consists of a series of parallel lines. However, the spacing of the lines of the two alignment patches is slightly different, thus giving rise to an interference pattern. When the alignment patches have been printed, the operator manually inspects them to determine the position in the overlying alignment patches of the maximum or minimum ink density. From this information, the relative offset between the two printheads in the media feed direction may be determined.
Once this determination has been made, the processor of the printer compensates for any offset in the media feed direction between printheads by avoiding using those nozzles in each printhead that extend in the media feed direction beyond the nozzles of the other printhead. The processor of the printer also resets the xe2x80x9clogical zeroxe2x80x9d in terms of the nozzles"" numbering in each printhead. That is to say that the nozzles which are to be used in each printhead are re-numbered, where necessary, such that the nozzles in each printhead which correspond in terms of their position along the media feed direction are allocated the same number, in order to ensure correct registration between the images printed by the different printheads. In this manner, the print output of the two printheads may be aligned at the expense of a slightly reduced number of usable nozzles.
This technique suffers from the disadvantage that it is relatively slow, being non-automated and reliant upon an operator. Furthermore, the process is less suitable for use in printers having more than two printheads, due to the increased difficulty of determining the relative offsets for a greater number of printheads.
A second type of known system is generally used on large format ink jet printers, which employ separate printheads for each ink colour. In order to ensure that no misregistration occurs between the images formed by the different coloured ink drops on the print medium, an alignment routine is performed.
In this routine, alignment patches are printed across the sheet of print media with each printhead so that they are approximately aligned along the scan axis; i.e. in a direction perpendicular to the media feed direction. The positions of the alignment patches in the media feed direction are then measured using an optical scanner, often referred to as a line scanner, which is mounted on the printer carriage. This is achieved for each alignment patch by positioning the line scanner at the appropriate point along the scan axis so as to be able to detect the alignment patch and then feeding the print media backwards (i.e. in a reverse feed direction) so that the position of the patch on the media in the media feed direction may be determined. The line scanner is then positioned at the appropriate point along the scan axis to detect the next alignment patch and the print media is fed forwards once again in readiness for determining the position of the next patch in the media feed direction. Once the position of each alignment patch in the media feed direction has be determined in this manner, the relative offsets in the media feed direction between the individual printheads are calculated.
The print output of the different printheads are then aligned in the media feed direction in the same manner as described above with respect to the first type of prior art system; i.e. by avoiding using those nozzles in each printhead that extend in the media feed direction beyond the nozzles of the other printheads and by resetting the xe2x80x9clogical zeroxe2x80x9d in terms of the nozzles"" numbering.
Although this system functions satisfactorily, the process which it employs is relatively slow, since the print media must be fed backwards and then forwards again in order to measure the position of each of the alignment patches. As the trend for increased numbers of printheads in a printer continues, the duration of such an alignment procedure is proportionally increased. Additionally, this system suffers from a further problem in that it can only be used with printer mechanisms that are capable of feeding the print media in both a forwards and a reverse feed direction. Thus, this technique is generally not applicable to printers in which the reverse feed direction of the media feed motor is used to perform other functions, such as powering a duplexing mechanism. Such printers include many high production, small format printers.
It would therefore be desirable to provide a system and method for determining a relative offset in the media advance direction between the printheads of a printer, which overcomes one or more of the disadvantages associated with the prior art.
According to a first aspect of the present invention there is provided a method of determining a registration offset in a hard copy apparatus, comprising the steps of: marking a alignment pattern on a print medium with a first pen; traversing the pattern in a first direction with a sensor and measuring the position of a portion of the pattern in the first direction; and, determining the offset of the pattern in a second direction, the pattern being configured such that the measured position in the first direction is indicative of a registration offset in a second direction.
By using an alignment pattern that is configured such that a measurable distance associated with the pattern in a first direction, for example along the scan axis of a printer device, allows the placement of the pattern in a second direction, for example along the media feed direction of the printer device, to be determined several advantages are realised.
Firstly, the alignment pattern may be printed and then scanned in the same direction, for example, along the scan axis direction of a printer. Thus, the two processes may be implemented without having to feed the print media, or having to scan the alignment pattern in a direction different from that in which the alignment pattern was printed. Thus, complex scanning arrangements may be avoided.
Moreover, this makes it possible to avoid the necessity associated with some prior art methods of requiring the alignment patterns, once printed, to be moved backwards and forwards under an optical scanner in order to establish their position along the media feed axis. As a consequence, the process by which the printheads offsets in the media feed direction may be achieved according to the present invention is comparatively rapid. This is because one pass of an optical scanner across the print medium may be sufficient to measure offsets of even a large number of printheads in the media feed direction.
Preferably, the alignment pattern of the present invention comprises two lines, one arranged parallel to the media feed axis and a second arranged at 45 degrees to the first. By scanning a narrow path across the scan axis of the media, intersecting both lines, the distance between the two points in the scan path intersected by the two lines may be measured. Due to the fact that the two lines of the alignment pattern are arranged at 45 degrees to each other, the measured distance will be equal to the perpendicular distance from the scan path to the point at which the two lines intersect. Thus, a change in the offset of a printhead in the media feed axis will cause the position of the alignment pattern, including both lines, to be offset relative to the scan path. Therefore, the distance between the two points in the scan path intersected by the two lines will change in proportion to the offset. Thus, by measuring the distance between the point in each line intersected by the scan path, the offset of the printhead in the in the media feed axis may be determined.
Preferably, the method also includes the step of compensating the measured registration offset for any errors introduced into the measurement process by a non-constant pen-to-paper spacing in the region of the alignment pattern. According to a preferred embodiment of the present invention, this is achieved by additionally printing two or more reference patterns, with a further pen, in known positional relationships relative to the alignment pattern. The reference patterns are printed with a single printhead in order that no significant offset between the reference patterns exists in the media feed direction. The reference patterns are configured in a similar manner to the alignment pattern, in that a measured position or distance in the first direction is indicative of a registration offset in a second direction. By determining what difference, if any, lies between the respective positions of the reference patterns in the second direction, an estimation of the error introduced into the measurement process by a non-constant pen-to-paper spacing in the region of the reference patterns may be obtained. The error in the position of the alignment pattern may then be determined by interpolation.
Advantageously, this method also provides for a correction for any errors introduced into the offset measurement process that might be caused by skewing of the print media between the steps of printing and scanning the alignment pattern. Thus, this embodiment makes the invention highly suited to printer devices which have a scanner located at a different point on the media path to the printheads; for example downstream.
The present invention also extends to the corresponding apparatus for implementing the above method. Furthermore, the present invention also extends to a computer program, arranged to implement the method of the present invention.